**Core Concept**
Corynebacterium diphtheriae is a pathogenic bacterium that requires specific culture conditions to grow. It is an aerobic, non-motile, gram-positive rod that can be isolated on selective media.

**Why the Correct Answer is Right**
Corynebacterium diphtheriae grows best on Loeffler's serum slope, which is a type of enriched medium. This medium provides the necessary nutrients, including blood and serum, to support the growth of the bacterium. The serum slope allows for the rapid growth of C. diphtheriae, enabling the identification of the organism within a short period. The characteristic metachromatic granules in the cells can be visualized using Albert's stain.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood agar is not the primary medium for C. diphtheriae culture, as it does not provide the necessary nutrients for rapid growth.
**Option B:** McConkey agar is a selective medium for gram-negative bacteria, particularly those that ferment lactose, and is not suitable for C. diphtheriae.
**Option C:** Sabouraud dextrose agar is a medium for fungal cultures and is not used for the isolation of C. diphtheriae.

**Clinical Pearl / High-Yield Fact**
Corynebacterium diphtheriae can produce potent exotoxins that cause diphtheria, a potentially life-threatening infection. Early diagnosis and isolation on Loeffler's serum slope are crucial for the control of outbreaks.

**Correct Answer: C. Loeffler's serum slope.**